In a device disclosed in Japanese Unexamined Patent Publication No. S63-165735, a beam emitted from one end face of a laser is detected through a gas, which is present in a measuring tank, by a wave detector, and a beam emitted from the other end face of the laser is detected through a reference gas, which is present in a reference tank, by another wave detector. According to this device, since signals output from the two wave detectors are simultaneously acquired, differential optical absorption spectroscopy measurement is performed for the gas present in the measuring tank and the gas can be analyzed.